Add support for displaying decrypted messages

CMakeLists.txt

deps/CMakeLists.txt

include/Cache.h

include/ChatPage.h

include/Logging.hpp

include/MainWindow.h

include/MatrixClient.h

include/Olm.hpp

include/timeline/TimelineView.h

src/Cache.cc

src/ChatPage.cc

src/CommunitiesList.cc

src/Logging.cpp

src/MainWindow.cc

src/MatrixClient.cc

src/Olm.cpp

src/RoomList.cc

src/main.cc

src/timeline/TimelineView.cc